Need help: O'Caml C Interface

[Frank Christoph <christo@nextsolution.co.jp>]

I have a C function 

  void getCharWidth(int code, double *x, double *y);

which returns its values in the two pointer arguments.  I tried to write a
stub function that creates a 2-tuple and passes it back:

  external width : int -> float * float = "ocaml_getCharWidth"

via:

value ocaml_getCharWidth(value code)
{
  double x, y;
  Push_roots(r,1);

  r[0] = alloc_tuple(2);
  getCharWidth(Int_val(code),&x,&y);
  Store_double_val(Field(r[0],0),x);
  Store_double_val(Field(r[0],1),y);
  Pop_roots();
  return r[0];
}

where I used Store_double_val because, as far as I can see, there is no
"Val_double" macro.  However, this gives me a segmentation fault.  I'm not
really sure I understand how the *_roots functions work.  Do the two local
variables x and y need to be declared as of type value?  If so, how can I
pass them to getCharWidth (since Double_val doesn't return an lvalue)?  What
am I doing wrong?

------------------------------------------------------------------------
Frank Christoph                 Next Solution Co.      Tel: 0424-98-1811
christo@nextsolution.co.jp                             Fax: 0424-98-1500